When it comes to solar power, not all roofs are created equal. Some face the wrong way, are angled poorly, fall under a tree’s shade or abut a taller building. And some have everything right but the location, location, location.

The number of new photovoltaic installations in the United States doubled in 2010. About a third of them were residential systems, reports the Solar Energy Industries Association.

How did homeowners and companies know if their property was well suited for solar? They likely relied on advice from contractors, but work has been underway to give us quicker previews into our buildings' solar power potential. Swedish researchers recently developed a GIS-based tool, called SEES (Solar Energy from Existing Structure), to calculate the solar irradiation of specific areas.

Integrating geographical and climate data with the physical details of a space—roof angle, shadows cast from nearby structures or trees—SEES assesses how much sunlight a location would likely receive over the course of the year. A color-coded map (right) shows in three dimensions the best spots to place PV panels. The predictions might be as small as a particular corner of one side of your roof or cover a whole town’s solar strategy.

We have used Gothenburg as pilot town in the project, but the method can be used in all municipalities where the necessary data is made available. The users can judge the suitability of a roof for solar voltaic panels or solar thermal panels across a wide range, based on this.

Companies and municipalities can access the system for free in Sweden, but so far details are hazy on whether it will be available elsewhere. Earlier this year, the University of California San Diego created its own Google Earth-based calculator that suggests the best angle and tilt for panels in order to maximize solar performance. Unfortunately for most of us, it only covers southern California. These scientists also take into account how strong sunlight hits a panel at a particular time of day. The ideal roof, they said, would soak up the sun at peak demand times. Unfortunately for most of us, the research only covers southern California.

The UCSD project does plan to expand its coverage. Until then, getting to know your roof is a good idea if you are in the market for solar. The National Renewable Energy Laboratory also offers an In My Backyard tool for estimating solar and small-scale wind power generation.